It has become increasingly necessary to improve the quality of cabin air delivered in passenger transport aircraft. Furthermore, aircraft component life requirements have been increased to the point where conventional seals used to isolate machinery oil and oil vapours from cabin air do not have sufficient life to meet the current oil-free environment standard.
For aircraft ground operations during passenger boarding and deplaning, as well as aircraft taxiing, takeoff and landing, the APU conditions cabin air for the support and comfort of passengers and crew members. In connection with a typical pneumatic environmental control system (ECS) that is used on aircraft, the APU is a small gas turbine engine that drives a load compressor and electric power generator. The load compressor pumps atmospheric air to sufficient pressure that when cooled and expanded it conditions hot and humid ambient air to a comfortable cabin air temperature and humidity. Similarly, the compressed air can be controlled to warm the cabin if ambient air is too cold for comfort in the cabin.
Since the APU has oil-lubricated bearings, steps must be taken to assure that lubrication oil does not mix with cabin air supplied by the APU. Labyrinth seals, controlled (radial clearance) gap seals and face seals are currently used in conjunction with sealing air to prevent oil migration from the gearbox and bearings of the APU into the load compressor air supply and ultimately the aircraft cabin air. Unfortunately, the face type seals wear at such a rate that the lubrication oil for the APU gears and bearings eventually migrates into the cabin air between normally scheduled APU overhauls.